Use of C0 Controls

We could (and probably should) just ignore this and let the rendering of these characters be system/font specific.

However HTML5 allows characters 11 and 12 to be used directly as character data (line tabulation and form feed) and specifies that they are "space characters"
that leads to the question raised by Henri Sivonen on the public list what the
rendering should be here, should we state that these two characters take part in
the white space normalisation that we apply to token elements (replacing runs of white space by a space character (32)